This project aims to characterize the role of BRCA1 associated protein-1 (BAP1) in normal and malignant hematopoiesis. BAP1 has been found to be mutated and downregulated in human myelodysplastic syndromes (MDS) consistent with BAP1 functioning as a novel tumor suppressor in the hematopoietic compartment. The mechanism(s) of action and the context in which BAP1 acts remains largely unknown. Our collaborators have recently shown that mice with whole body conditional loss of BAP1 manifest pancytopenia which is a characteristic of human MDS. Previous work by our lab and others has demonstrated that BAP1, an enzyme that has a role in deubiquitination of histone H2A at lysine 119, forms a ternary complex with ASXL1. ASXL1 is commonly mutated in human MDS and acute myeloid leukemia (AML) and ASXL1 mutations are predictors of poor patient outcome. We have shown that while BAP1 and ASXL1 do interact in hematopoietic cells, these proteins have independent effects on target gene expression and global histone regulation. Whereas loss of ASXL1 results in upregulation of expression HOXA cluster and decreased Polycomb Repressive Complex 2 (PRC2) function, BAP1 depletion increases global H3K27me3 and represses HOXA gene expression. My central hypotheses are: (1) Genetic deletion of BAP1 in the hematopoietic specific compartment causes abnormal hematopoiesis due to a stem and/or progenitor cell defect, (2) BAP1 regulates the PRC2 complex and MDS patients with BAP1 downregulation will be sensitive to EZH2 inhibitors, and (3) BAP1 could collaborate with other oncogenic alleles including ASXL1 due to dysregulation of the chromatin state in hematopoietic stem and progenitor cells. My long term goals are to understand MDS disease pathogenesis and mechanisms by which MDS transforms to AML and to identify novel targeted therapeutics to treat human MDS. The first specific aim will be to characterize the stem and progenitor cell phenotype following BAP1 hematopoietic-specific loss in a novel mouse model in addition to characterizing the in vivo epigenetic phenotype of BAP1 loss in hematopoiesis. The second specific aim will serve to identify mechanisms by utilizing ASXL1 and EZH2 conditional knockout models and in vitro BAP1 shRNA systems as well as to assess the sensitivity of BAP1 loss-of-function model systems to epigenetic therapies. The implications are that these findings will be relevant in other cancer contexts where BAP1 mutations are quite prevalent, such as uveal melanoma, mesothelioma, and renal cell carcinoma. Further, we will establish novel MDS models for future testing of novel therapeutic agents in subsequent studies.